Purpose of this Supplement

At the time ARINC Characteristic No. 545 was finalized the altitude reporting project was just getting under way and therefore, not all of the basic requirements for the digitized output, the protective circuitry associated with that output, and the encoding scheme to be employed, had been completely firmed up. Furthermore, it had been expected that all air data system manufacturers would employ some form of disc encoder for the altitude reporting output and thus, a specification on marginal voltage limits for the digitized output were not deemed necessary. As the program of altitude reporting moved ahead a number of basic system decisions were finalized by the ICAO VII COM Division Electronic Engineering Committee in developing an Altitude Computer System, which has now been issued as ARINC Characteristic No. 549, revealed several deficiencies in the requirements of ARINC Characteristic No. 542.

Most of the points relating to the compatibility with ARINC Characteristic No. 549 were discussed with equipment manufacturers during the development of the ARINC Characteristic No. 549 document and thus, it is believed all air data systems already provide the features set forth in this Supplement. Thus, for these particular items this Supplement will simply confirm what is now recognized as being desirable by the set manufacturers

With respect to the ICAO VII COM Decision decisions on an altitude encoding format, this information has previously been circulated and is already understood by the equipment manufacturers.

This Supplement No. 1 will therefore reflect the changes necessary to bring the original Characteristic No. 545 up-to-date with respect to the ICAO decisions and to make the original Characteristic No. 549 with respect to the digitized altitude output circuit.